Statistics of Penetration Depth of Photons Re-emitted from Irradiated Tissue

We derive some statistical properties of photons emitted from bulk tissue that has been irradiated by a continuous beam of laser light. The properties include the distribution and moments of the depth reached by a photon, conditional on its having reached the surface at a (dimensionless) distance, ϱ, from the point of entry of the beam into the tissue. Theoretical predictions are made on the basis of a lattice random-walk model and are checked by simulations of the light penetration problem for a more realistic continuum model.